In the field of petroleum well drilling and logging, resistivity-logging tools are frequently used to provide an indication of the electrical resistivity of rock formations surrounding an earth borehole. Such information regarding resistivity is useful in ascertaining the presence or absence of hydrocarbons. A typical resistivity-logging tool includes a transmitter antenna and a pair of receiver antennas located at different distances from the transmitter antenna along the axis of the tool. The transmitter antenna is used to create electromagnetic fields in the surrounding formation. In turn, the electromagnetic fields in the formation induce an electrical voltage in each receiver antenna. Due to geometric spreading and absorption by the surrounding earth formation, the induced voltages in the two receiving antennas have different phases and amplitudes.
Experiments have shown that the phase difference (Φ) and amplitude ratio (attenuation, A) of the induced voltages in the receiver antennas are indicative of the resistivity of the formation. The formation region (as defined by a radial distance from the tool axis) to which such a resistivity measurement pertains is a function of the frequency of the transmitter and the distance from the transmitter to the mid-point between the two receivers. Thus, one may achieve multiple radial depths of investigation of resistivity either by providing multiple transmitters at different distances from the receiver pair or by operating a given transmitter at multiple frequencies, or both.
In addition to varying depths of investigation, resistivity logging tools may be provided with a directional sensitivity by tilting one or more of the transmitter and receiver antennas as described in U.S. Pat. No. 7,265,552 to Michael Bittar. As explained in the Bittar patent, if the resistivities corresponding to the various rotational orientations are different, such differences often indicate the direction of a boundary between formations having different resistivities. In directional drilling, it is often the goal to steer the drill into a formation bed and to “follow” (to drill substantially parallel to) one of the boundaries of the bed to maximize the extent of the borehole within the bed. When implemented as logging-while-drilling (“LWD”) tools, the directional resistivity-logging tools provide valuable measurements for such geosteering applications.
Existing resistivity measurement tools for geosteering provide measurement ranges of up to 6 m (20 ft), which may not be enough in many situations. Moreover, such tools may not perform well in the oil fields of Saudi Arabia, where resistivity logs exhibit poor contrasts. Halliburton has recently proposed an acoustic measurement technique as an alternative basis for geosteering. The acoustic tool may provide a measurement range of up to 15 m (50 ft).
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.